A new monoanionic dithiolene ligand is found in Tp*MoO(S(2)BMOQO). A combination of X-ray crystallography, electronic absorption spectroscopy, resonance Raman spectroscopy, and bonding calculations reveal that the monoanionic dithiolene ligand possesses considerable thiolate-thione character resulting from an admixture of an intraligand charge transfer excited state into the ground state wave function. The unusual dithiolene exhibits a highly versatile donor-acceptor character that dramatically affects the Mo(IV/V) redox couple and points to a potentially noninnocent role of the pterin fragment in pyranopterin Mo enzymes.